The present invention, in some embodiments thereof, relates to target detection and, more particularly, but not exclusively, to range-Doppler target detection.
A radar is a system that uses propagating waves to detect objects within a certain spatial volume. A radar system may be used to determine the range, altitude, direction, and/or speed of fixed objects, or objects that are moving through the spatial volume of interest.
The term “RADAR” was coined in 1941 as an acronym for Radio Detection and Ranging. This acronym, of American origin, replaced the previously used British abbreviation “RDF” (Radio Direction Finding). The term has since entered the English language as a generic word, “radar,” that has lost its original capitalization.
A radar system transmits electromagnetic waves into the spatial volume. If and when an electromagnetic wave comes in contact with an object in space, the electromagnetic wave is reflected and scattered by the object. Thus, a reflected signal is propagated back toward the radar system. The radar system receives the reflected wave and detects the object. A radar system is typically configured to transmit many pulses into the spatial volume every second. These pulses are received by the radar system. By processing these pulses the radar system can detect the presence of the object and determine if the object is moving, and if moving, its speed and direction.
One way to measure the distance between the radar system and the target is to transmit a short pulse of radiation, and to measure the time elapsed for the reflection to return. The distance is one-half the product of round-trip time and the velocity of the signal. This concept was exploited already in certain early radars. Since the velocity of the signal is the velocity of light, the round-trip time is very short for terrestrial ranging.
Modern radar systems employ the Doppler effect for determining the speed of the object. Conventional pulse Doppler radar systems radiate a coherent pulse train that, when reflected by a target, returns signals that can provide data including the range of the target (the distance from the antenna), and its radial velocity with respect to the antenna. Many Doppler radar systems have been developed for many applications including airborne radar systems, surface-based systems and marine systems.
With the rapid advances in digital signal processing (DSP) technology, many modern radar systems, including pulse Doppler radar systems, digitize the return signals and utilize DSP for target detection and discrimination. DSP technology can improve the performance of a radar system while reducing its cost. Furthermore, the flexibility allowed by DSP systems can improve signal detection by enabling real-time adaptation of the receiver to various conditions.
A radar receives echo which includes a target reflection signal and other reflection signals (which are called clutter), e.g., from the ground. Pulse Doppler radar systems are used where moving targets are desired to be detected amidst an environment replete with clutter. Target detection processing, which is known as standard processing of a radar, is generally aimed for suppressing the clutter by discriminating the target reflection signal from the clutter. For moving targets, the discrimination is based on the moving speed of a target. Typically, a filtering process is performed on Doppler frequencies generated in relation to the moving speed of the target.
A typical example of this filtering process for the Doppler frequencies includes Fast Fourier Transform (FFT). It is recognized that in order to obtain a signal process gain, the transmission frequency at the time of receiving the signal should be coherent and transmission pulse intervals should be constant.
Known radar systems employ a mechanically rotated antenna. The beam radiated by the antenna is propagated into space along the antenna boresight. The spatial volume is, therefore, scanned by rotating the antenna, typically in a 360° sweep. A target search by the radar is typically performed by sequentially transmitting transmission pulses while rotating the antenna. Because the antenna is rotated, a time period for irradiating a target with the transmission radio waves is limited. The number of transmission pulses to be transmitted within this irradiation period is called a hit number.
The above-described Doppler filtering process is performed on transmission pulses within a time period called a coherent processing interval (CPI) having the hit number as an upper limit.
Also known are phased array radar antennas which include a plurality of antenna elements disposed in a two-dimensional array. These antenna elements are used for both transmission and detection of electromagnetic energy in an alternating fashion. A phased array radar system does not require moving parts, but may have them. For example, a planar array may be rotated mechanically to cover a required azimuthal range. However, a phased array radar does not require mechanical steering; it can be steered through phase shifting, or time delaying, signals to the various elements. A phased array radar beam is emitted by the plurality of elements using a principle known as superposition whereby the waves emitted by each element in the phase array are combined. The amplitudes and phases of the waves constructively and destructively interfere with each other to create a composite radar beam having a predetermined radiation pattern. By continuously varying the amplitudes and phases of the waves being emitted from the various elements of the array, the composite radar beam may be pointed in a certain direction, or be made to scan back and forth (in azimuth) or up and down (in elevation). Thus, a phased array antenna propagates a single beam into the spatial volume and the reflected return signals are received by all of the elements in the phased array. Accordingly, a phased array radar system may be viewed as a Single-Input Multiple Output (SIMO) system because the antenna array transmits a single composite radar beam and the reflected signal is received by all of the elements in the phase array.
Another type of radar system is the so called Multiple-Input Multiple-Output (MIMO) radar system. A MIMO radar system employs multiple independent transmitters and multiple receivers that are configured to take advantage of the geometry of the transmit and receive locations to increase target resolution. In some MIMO radar systems, each transmitter employs an omni-directional antenna having a low gain. In such systems a desired signal to noise ratio (SNR) on a given target can be achieved using a longer coherent integration time, resulting in enhanced Doppler resolution. The multiple transmitter elements in a MIMO radar system transmit orthogonal waveforms. Having each transmitter direct an orthogonal signal into the search volume allows each receiver to distinguish the transmission source of a received reflected signal. On the receive side, each receiver element is configured to accommodate each orthogonal signal, typically by employing a matched filter for each orthogonal signal.
Also known is the use of Compressive Sensing (CS) instead of matched filter [Herman and Strohmer “High-Resolution Radar via Compressed Sensing,” IEEE Trans. on Signal Proc, Vol 57, No. 6, 2009].
CS has also been used in the context of step frequency waveform which requires transmission of pulse train [Shah et al., “Step-Frequency Radar with Compressive Sampling (SFR-CS)”, in Proc. ICASSP 2010, 2010], and in MIMO radar, wherein the antenna array elements transmit and receives uncorrelated waveforms [Yu et al., “MIMO Radar Using Compressive Sampling,” IEEE Journal on Selected Topics in Signal Proc., Vol. 4, no. 1, 2010].
It has been suggested that CS may reduce the sample rate and the number of antenna elements [Ender, J. H. G., “On compressive sensing applied to radar,” Signal Processing. Vol. 90, Issue 5, 2010, pp. 1402-1414].
Additional background art includes Baraniuk and Steeghs, “Compressive radar imaging,” in Proc. Radar Conf., 2007, pp. 129-133; and Potter et al., “Sparsity and compressed sensing in radar imaging,” Proceedings of the IEEE, Special Issue on Applications of Compressed Sensing. Vol. 98, no. 6, June 2010.